1. Field of the Invention
The present invention generally relates to a method of image data interpolation and, more particularly, is directed to a method of image data interpolation used for converting a field signal into an interlaced signal in an image signal processing apparatus.
2. Description of the Prior Art
In a video tape recorder (VTR), a video disk apparatus or an image signal processing apparatus using a frame memory, an interlaced signal is generally obtained by using an image signal of only one field. As a first example of this method of obtaining an interlaced signal, there has been proposed a method of obtaining an interlaced signal of two fields by using the same field signal. In this example, when an odd field signal shown in FIG. 1A, for example, is supplied, the same signal as the odd field signal is outputted as an even field signal as shown in FIG. 1B. FIG. 1C shows a diagram illustrating lines of FIG. 1B seen from the lateral direction.
As a second example of the above-described method, there has been proposed a method of interpolating data by using a mean value of a given field signal (hereinafter referred to as mean value interpolation method). FIG. 2 shows a block diagram of a mean value interpolation circuit for realizing the mean value interpolation method. Referring to FIG. 2, an odd field signal is applied to a one-horizontal line delay circuit 2 and also to a divider circuit 4 for dividing an input signal by a factor of two, then the output signal of the delay circuit 2 is applied to another divider circuit 6 for dividing by a factor of two. The output signals of the divider circuits 4 and 6 are added by an adder 8, and then the input odd field signal and the output of the adder 8 are alternately outputted as an output signal X in response to an even or odd signal, respectively. Thus, another field signal is obtained as shown in FIG. 3B by obtaining a mean value of two data of contiguous two odd field signals shown in FIG. 3A. FIG. 3C is a diagram illustrating lines of FIG. 3A seen from the lateral direction. Accordingly, an interlaced signal is obtained by using the image signal of only one field.
However, the above-described first example of the conventional interpolation method has the disadvantage such that a line flicker is generated between an even line a and an odd line b shown in FIG. 1B, so that a slant line appears to be broken and V-jitter configuration. That is, a portion having a high-frequency component where the luminance or level of an image changes abruptly in the vertical direction is returned by an interlaced carrier to cause a flicker of 30 Hz. This means that a center of a point where the level of an image changes in the vertical direction fluctuates depending on the field numbers.
Also, in the second example of the conventional interpolation method, a slant line appears to be smooth and the generation of the line flicker is less when compared with the first example. However, the image signal thus obtained through the mean value interpolation processing is that filtered by a vertical low-pass filter (LPF) having a dip point at a 525/2 line, so that apparent sharpness is degraded. Further, at a portion where the luminance of an image changes finely in the vertical direction such as a fine stripe extending to the horizontal directions, a flicker will appear at a large area. As a typical example, when the mean value interpolation processing is performed as to an image signal whose image of an odd field changes at every line as shown in FIG. 4A, a field signal generated by the processing will have a large DC component as shown in FIG. 4B and will be quite different from the image of the given field as shown in FIG. 5. FIG. 4C is a diagram illustrating the lines of FIG. 4B seen from the lateral direction wherein data of an image of an even field is equalized in level by the adjacent data of an odd field.